I'm on my second business that was started while having a full-time job. The first failed, but it did not hurt me or my career because we iterated its development using a million little tests. So when we decided to move on from the venture, we did it without risking too much.

For the second business, we expanded the “million little tests” approach.

We started off by testing the idea — validating the problem actually exists and that people cared about a solution. But we still had to figure out the how. So we came up with a variety of solutions and just talked to people about them. It's free, and it’s easy (except for the psychological vulnerability of sharing raw ideas and facing rejection for the majority of the time). If you want to, you can also set up a quick/cheap landing page and buy some Google/Facebook ads to send traffic and see if anyone cares. And after all of this you still have your day job.

Then create mock-ups — pen and paper or iPad drawings. The costs of this are very minimal. And you can show them to people and get feedback. Then you’ll want to create a prototype — a Keynote/PowerPoint or Photoshop/Sketch. At this point, the most money you're out is for the software, but that’s all. And then get feedback.

If you’ve done all of this, you should have a pretty good understanding of whether you're onto something attractive, or your idea still needs refinement. But only at this phase do you decide if you're going to invest real money into developing it. And the development process takes a long time, too, but it can (and should) be iterative as well. That means you’ll need to do more tests along the way.

Then, you have what I believe is the biggest test to date: Beta-testing. You won't nail it the first time around (or even the second or third, for that matter), but you'll be closer than you were before all of the earlier testing. Now people actually EXPERIENCE your product, and you test and adjust it accordingly. If, at this point, you feel confident, THEN you can decide to quit your full-time job. You can also launch and wait for traction before quitting, but some business models will require full-time work at or before this point.